ci(vfs): install the GCC-compatible Rust target before building

The Windows runners used by `vfs-functional-tests.yml` ship `rustup`
plus a `*-pc-windows-msvc` default toolchain (see
https://github.com/actions/runner-images/blob/main/images/windows/Windows2022-Readme.md
and
https://github.com/actions/partner-runner-images/blob/main/images/arm-windows-11-image.md),
but no precompiled `std` for `*-pc-windows-gnu` or
`*-pc-windows-gnullvm`. With the Makefile now picking a
GCC-compatible target triple based on `$(MSYSTEM)`, the build step
needs that precompiled `std` to be installed before invoking `make`,
otherwise `cargo build --target <triple>` fails to find a usable
`std` for the chosen target.

Add a step between the SDK setup and the `make` invocation that
selects the matching triple from `$MSYSTEM` (which
`git-for-windows/setup-git-for-windows-sdk` exports for every
subsequent step) and runs `rustup target add` for it. The mapping
mirrors what `config.mak.uname` derives from `$(MSYSTEM)` and
`$(HOST_CPU)`, just enumerated explicitly here since CI has direct
knowledge of which MSYS2 subsystems the matrix actually exercises
(`CLANGARM64` for the ARM64 runner, `MINGW64` for the x86_64
runner).

For a `staticlib` crate-type `cargo build` does not invoke an
external linker, so no further toolchain components (e.g. the
`gnullvm` LLVM linker) need to be installed; `rustup target add`
alone is sufficient.

Assisted-by: Claude Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/vfs-functional-tests.yml

@@ -50,6 +50,27 @@ jobs:
           exec $cmd "$@"
           EOF
 
+      - name: Install GCC-compatible Rust target
+        shell: bash
+        run: |
+          # The hosted Windows runners ship a rustup-managed Rust whose
+          # default toolchain targets the MSVC ABI. That produces a
+          # `gitcore.lib` which the MinGW GCC used by the rest of the
+          # build cannot link. Install the precompiled `std` for a
+          # GCC-compatible target triple matching the MSYS2 subsystem;
+          # the Makefile selects the same triple via $(MSYSTEM) and
+          # passes it to `cargo build --target`.
+          case "$MSYSTEM" in
+          CLANGARM64) target=aarch64-pc-windows-gnullvm ;;
+          CLANG64)    target=x86_64-pc-windows-gnullvm  ;;
+          CLANG32)    target=i686-pc-windows-gnullvm    ;;
+          UCRT64)     target=x86_64-pc-windows-gnullvm  ;;
+          MINGW64)    target=x86_64-pc-windows-gnu      ;;
+          MINGW32)    target=i686-pc-windows-gnu        ;;
+          *) echo "::error::Unsupported MSYSTEM: $MSYSTEM"; exit 1 ;;
+          esac
+          rustup target add "$target"
+
       - name: Build and install Git
         shell: bash
         env: